The "strings" of celery are composed of sclerenchyma tissue.

Sclerenchyma Tissue

A supportive plant tissue composed of thick-walled, usually dead cells, offering structural support and protection.

Strings

Objects composed of fibers twisted or braided together, or a fundamental concept in programming representing sequences of characters.

Celery

A plant of the family Apiaceae, commonly used as a vegetable, known for its crunchy stalks.